Compare the request with the result.
A feedback loop compares a target with a measurement or estimate. The difference, called error, influences the action taken to bring the result towards the target.
For a speed loop, the target is requested speed and the feedback represents actual speed. A position loop instead compares positions.
Follow one correction cycle.
- CompareRequested speed is compared with measured speed.
- ActThe controller adjusts the motor torque demand.
- Measure againNew feedback shows the result of that action.
A load change creates a new demand.
A speed controller can increase torque demand to oppose the disturbance, within the motor and drive’s available limits.
Correction takes time. Feedback cannot create unlimited torque, remove mechanical problems or make an unsuitable system follow any command.
Control often uses nested loops.
An outer position loop may request speed. A speed loop then requests torque-producing current, and an inner current loop controls electrical behaviour. The exact arrangement depends on the drive.
A lift controller can plan the journey while the drive handles fast motor control. These functions work together but are not all necessarily in one device.
Good feedback is part of the complete system.
Sensor quality, timing, mechanical coupling and the control model affect the result. Excessive correction or delay can produce oscillation rather than smooth tracking.
Motion feedback is not automatically safety-rated monitoring. Door permissions, brake monitoring and protective functions require their own intended design and validation.
A closer look
“Sensorless” motor control can estimate speed or rotor conditions from electrical measurements and a motor model. It does not mean the drive has no information about the motor, nor that its behaviour is identical to encoder feedback in every operating region.
Check your understanding
If feedback shows the lift is slower than requested, can the drive always correct it immediately?
Reveal the explanation
No. Correction is limited by available torque, response time and the physical system. A fault or excessive demand may prevent the target being reached.